Geothermal wells may produce dry steam from vapor-dominated reservoirs or a mixture of steam and hot water (brine) from liquid-dominated reservoirs in which the hot brine flashes partly to steam as the brine flows upwardly from the well through a well bore. The steam produced by both types of geothermal resources contain noncondensable gases herein defined as carbon dioxide gas with varying amounts of other constituents, such as methane, nitrogen, hydrogen, argon, ammonia and hydrogen sulfide, which are not condensable at ambient conditions, i.e., which do not condense at the operating pressures and temperatures of geothermal power plants.
Geothermal energy is made available to the marketplace by converting it to electricity. To accomplish this, geothermal steam is fed to a steam turbine that drives an electric generator. Exhaust steam from the turbine may be discharged directly to the atmosphere or, for greater thermal efficiency, the steam may be condensed to reduce the pressure at the exhaust outlet of the turbine. The noncondensable gases that accumulate in the condenser are extracted and vented to the atmosphere.
If the turbine is located in an environmentally sensitive area, or if the geothermal steam contains a high concentration of hydrogen sulfide gas, it may be required that the hydrogen sulfide gas be first removed from the noncondensable gases before the gases are discharged to the atmosphere. The reason for this is that hydrogen sulfide gas is toxic at high concentrations. Even at low concentrations, hydrogen sulfide gas presents a problem, mainly aesthetic, because the hydrogen sulfide gas imparts a "rotten egg" smell that can be detected by the sensitive human nose at concentrations as low as 0.5 parts per million.
The Stretford process has often been used to treat noncondensable gases before they are discharged to the atmosphere. The Stretford process uses a vanadium catalyst to convert the hydrogen sulfide gas to elemental sulfur, thereby removing hydrogen sulfide gas from the noncondensable gases. The Stretford process is effective in removing the hydrogen sulfide gas, but the processing unit needed to carry out the Stretford process is relatively expensive and is costly to install, and the elemental sulfur product that is recovered is often contaminated and must be discarded as waste material.
The Dow Chemical Company has developed a process in which hydrogen sulfide gas is incinerated and converted into a water soluble thiosulfate. This liquid material is then disposed of by injecting the wastes through a well into the reservoir from which the geothermal fluid was produced. The Dow process is claimed to be less expensive than the Stretford process, and it does not produce solid waste material that requires special handling and disposal because of the contaminants. However, reactive thiosulfate may break down in the hot environment of the reservoir and form sulfates which may react with the calcium and magnesium values contained in the rock matrix of the reservoir. Such reactions will form precipitates that may plug or reduce the permeability of the waste water injection zone.